1. Field of the Invention
This invention relates generally to a technique for applying a protective coating to a eutectic or near-eutectic alloy, and more particularly to a technique for providing a firmly adherent protective coating to a eutectic or near-eutectic alloy.
2. Description of the Prior Art
It is well known to protect and prolong the useable life of metal bodies by treating their surfaces with one or more protective coatings. In fact, a wide variety of surface coatings and coating techniques have been developed in the prior art for just this purpose, and these have been met with varying degrees of success and acceptability dependent upon the particular metal being treated and the particular coating composition.
In the usual instance, unless the applied coating is quite thin, the coating material must be selected so that its coefficient of thermal expansion matches identically or nearly identically with that on the metal being protected. If the coating is too thick and if the coefficients differ by too great an extent, poor adhesion and peeling of the coating can occur, which problem becomes even more severe if the surface is subjected to frequent, or even occasional, thermal shocks. When the metal being protected is an alloy, such as a eutectic alloy, containing multiple crystalline phases, each phase will often have a somewhat different coefficient of thermal expansion. Therefore, it can become quite a difficult matter to identically, or even closely, match the thermal expansion coefficients, so that in such alloys coating adhesion has been an especially acute problem.
Another difficulty with the multiple phase eutectic alloys is that wear and corrosion can occur, not only on the top surface, but also within the upper stratum of the alloy, at the grain boundaries, or in those crystalline phases which have low resistance characteristics. With these types of alloys, merely applying a coating to the alloy surface may not provide sufficient protection for both the surface and the sub-surface from corrosion and wear.